1. Field of the Invention
The present invention relates to a switching power supply apparatus provided with a converter, such as a DC-DC converter or an AC-DC converter.
2. Description of the Related Art
FIG. 9 shows an example of a circuit construction of a converter which is a basic circuit element of a conventional switching power supply apparatus. The converter shown in FIG. 9 is an isolated and has a transformer 1 therein. A primary winding N1 of the transformer 1 is connected to a primary circuit 2 while a secondary winding N2 thereof is connected to a secondary circuit (output circuit) 3.
The primary circuit 2 includes a primary switching element Q (a MOS-FET in the example shown in FIG. 9) and a resistor 4 and has an input thereof connected to an input power supply 5. The secondary circuit 3 includes diodes 6 and 7, a choke inductor 8, and a smoothing capacitor 10 and has output terminals 3a and 3b thereof connected to a load (not shown). The direct-current input voltage Vin of the input power supply 5 is applied across the primary winding N1 of the transformer 1 in accordance with on-off operations of the primary switching element Q, and an output voltage is produced from the secondary winding N2 in accordance with the input voltage Vin. This output voltage from the secondary winding N2 is rectified and smoothed by the secondary circuit 3, and a direct-current voltage Vout is applied across the load.
As shown in FIG. 9, the gate of the primary switching element Q is connected to a control circuit 11. A tertiary winding N3 is provided in the transformer 1, and this tertiary winding N3 is connected to a detection circuit 12. This detection circuit 12 includes diodes 13 and 14, a choke inductor 15 and a smoothing capacitor 16. These circuit elements form the same circuit as the secondary circuit 3. Since a voltage substantially corresponding to a voltage generated across the secondary winding N2 is generated across the tertiary winding N3, the voltage across the tertiary winding N3 is rectified and smoothed by the same type circuit as the secondary circuit 3, whereby a voltage corresponding to the output voltage Vout of the secondary circuit 3 is generated across the smoothing capacitor 16. In addition to the above construction, the detection circuit 12 includes a diode 17 and voltage-dividing resistors 18 and 19. The voltage across the smoothing capacitor 16 is voltage-divided by the voltage-dividing resistors 18 and 19, and the divided voltage is applied as a detection voltage of the output voltage Vout to the control circuit 11.
The control circuit 11 controls the on-off operations of the primary switching element Q so that the output voltage Vout, which is output based on the detection voltage from the secondary circuit 3, has a stable set voltage value. In FIG. 9, N1xe2x80x2 denotes a primary winding and reference numeral 20 denotes a diode.
As described above, the switching power supply apparatus shown in FIG. 9 performs stabilization control on the output voltage Vout of the secondary circuit 3 by causing the detection circuit 12 to indirectly detect the output voltage Vout of the secondary circuit 3, and by causing the control circuit 11 to perform switching control of the primary switching element Q based on the detection voltage.
However, in the circuit shown in FIG. 9, since the switching power supply apparatus is constructed such that the output voltage Vout of the secondary circuit 3 is indirectly detected, there has been a problem that the output voltage Vout deviates from a set output voltage value Vsp because of a change of the input voltage Vin.
This problem may arise for the following reasons. For example, there is a case in which the number of turns of the secondary winding N2 and that of the tertiary winding N3 are different. In this case, a voltage induced across the secondary winding N2 and a voltage induced across the tertiary winding N3 are different. Therefore, it is necessary to use devices individually having various different characteristics, such as an endurance voltage, a forward voltage, and a reverse current, as the diodes 6 and 7 of the secondary circuit 3 and the diodes 13 and 14 of the detection circuit 12. Even when the number of turns of the secondary winding N2 and that of the tertiary winding N3 are the same, in the same manner as described above. there are some cases in which devices having various different characteristics are used as the diodes 6, 7, 13, and 14.
When the input voltage Vin is changed, for example, the reverse current of each of the diodes 6, 7, 13, and 14 is changed. When the characteristics of each of the diodes 6, 7, 13, and 14 are different, differences in changes of the reverse currents occur among the diodes 6, 7, 13, and 14.
For example, when the input voltage Vin has a voltage value xe2x80x9caxe2x80x9d, circuit constants of the control circuit 11 and the like are supposed to be set so that the output voltage Vout having the set output voltage value Vsp is stably output in accordance with switching control operations of the control circuit 11 based on the detection voltage of the detection circuit 12. Even though the control circuit 11 is formed as such, when the input power supply 5 supplying the input voltage Vin having a voltage value xe2x80x9cbxe2x80x9d, which is different from the voltage value xe2x80x9caxe2x80x9d, is connected to the primary circuit 2, concerning a case in which the input voltage Vin has the voltage value xe2x80x9caxe2x80x9d and a case in which the input voltage Vin has the voltage value xe2x80x9cbxe2x80x9d, there is a deviation between the detection voltages of the detection circuit 12 to the output voltage Vouts in accordance with a difference in the change of the reverse current of each of the diodes in response to the change of the input voltage Vin.
Since the control circuit 11 controls the on-off operations of the primary switching element Q based on the detection voltage, the output voltage Vout is obtained having a voltage value which deviates from the set output voltage value Vsp. Therefore, the output voltage Vout having the set output voltage value Vsp cannot be stably output.
As the input voltage Vin increases, the deviation of the detection voltage of the detection circuit 12 to the output voltage Vout increases. Therefore, as indicated by the dashed lines in FIG. 2 and FIG. 4, the greater the input voltage Vin becomes, the greater the deviation of the output voltage Vout to the set output voltage value Vsp becomes.
Recently, the operating voltages in ICs and LSIs tend to be lower, and lowvoltage driven and high-current outputs are demanded. Because of this demand, there are some cases in which, instead of the diodes 6, 7, 13, and 14, a synchronous rectifier, such as a power MOS-FET, is used. In this case, when the input voltage Vin is changed, the drive voltage of the synchronous rectifier is changed, and the resistance of the on-resistor of the synchronous rectifier is changed, whereby the voltage drop of the synchronous rectifier is changed. Likewise, because of this, there arises a problem in that the output voltage Vout cannot be stably output due to the change of the input voltage Vin.
The present invention is made to solve the foregoing problems, and an object of the present invention is to provide a switching power supply in which an output voltage having a set voltage value can be stably output regardless of changes in the input voltage.
To achieve the foregoing objects, this invention provides a method for solving the above problems using the construction described below. That is, a first aspect of the invention provides a method for solving the above problems using a construction in which in a switching power supply apparatus is provided for outputting a voltage from the secondary winding of a transformer based on an input voltage in accordance with on-off operations of a primary switching element, and for rectifying, smoothing, and outputting the voltage by an output circuit; the switching power supply apparatus including a tertiary winding provided in the transformer; a detection circuit in which the output voltage output from the output circuit is indirectly detected using the tertiary winding, and the detected voltage is output; a compensating-voltage-superposition circuit which generates a compensating voltage, in accordance with the detection voltage, for compensating for a deviation of the detection voltage of the detection circuit to the output voltage of the output circuit in accordance with a change of the input voltage and which superposes the compensating voltage on the detection voltage output from the detection circuit; and a control circuit which controls the on-off operations of the primary switching element so that the output voltage of the output circuit is stabilized based on the detection voltage having the compensating voltage superposed thereon.
A second aspect of the invention is the provision of a switching power supply apparatus according to the first aspect of the invention and is constructed such that the compensating-voltage-superposition circuit generates the compensating voltage using a voltage output from the tertiary winding and superposes the compensating voltage on the detection voltage of the detection circuit.
A third aspect of the invention is the provision of the switching power supply apparatus according to the first aspect of the invention and is constructed such that the transformer is provided with a compensating-voltage-generation winding, and the compensating-voltage-superposition circuit generates the compensating voltage using a voltage output from the compensating-voltage-generation winding and superposes the compensating voltage on the detection circuit of the detection circuit.
According to this invention, in a case in which a switching power supply apparatus is constructed in which an output voltage is indirectly detected, since the device is constructed such that the compensating-voltage-superposition circuit is provided, the device generates a compensating voltage for compensating for a deviation of the detection voltage to the output voltage in accordance with the input voltage, performs compensation for the detection voltage by superposing this compensating voltage on the detection voltage, and applies the compensated detection voltage to a control circuit. The control circuit can thereby perform stabilization control of the output voltage so that the output voltage having a set voltage value is stably output based on the compensated detection voltage.
Thus, even though the input voltage may change, the output voltage having the set voltage value can be stably output without adverse influences of the changes of the input voltage, which enables the reliability of the switching power supply apparatus to be considerably improved.
In a switching device having a construction in which the compensating-voltage-superposition circuit generates the compensating voltage using a voltage across the tertiary winding, the compensating voltage can be output without more turns in the transformer. Therefore, an increase in the number of parts can be avoided.
In a switching device having a construction in which the transformer is provided with a compensating-voltage-generation winding, and the compensating-voltage-superposition circuit generates the compensating voltage using a voltage across the compensating-voltage-generation winding, since a voltage across the compensating-voltage-generation winding is sufficient to be generated in accordance with the input voltage, the voltage across the compensating-voltage-generation winding may be below the voltage across the tertiary winding. Compared with a case in which the voltage across the tertiary winding is used, reducing the voltage across the compensating-voltage-generation winding so that it is smaller than the voltage across the tertiary winding can decrease power consumption occurring in the compensating-voltage-superposition circuit. Because of this, the compensating-voltage-superposition circuit is constituted using low-voltage-endurance parts, which can prevent conductive loss in the compensating-voltage-superposition circuit from occurring. Therefore, reduction of the circuit efficiency of the switching power supply apparatus can be avoided.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.